There are home electrical appliances having single-phase induction motors for driving work-performing tools, for example such as electrical kitchen appliances, including food processors, blenders, choppers, beaters, whippers, grinders, slicers, and the like. In such appliances having work-performing tools it is desired that the tool stop rotating as quickly and smoothly as possible, whenever the A.C. electrical power is turned off or interrupted in any way.
Induction motors are often incorporated in such home appliances, because the induction motor is reliable and quiet and usually offers a long trouble-free, maintenance-free operating life. Moreover, single-phase induction motors have predetermined relatively narrow ranges of operating speed as determined by the number of magnetic poles in the stator. In other words their operating speeds, within their rated loading limits, are not greatly affected by the working loads imposed upon the driven tool. These operating characteristics of single-phase induction motors are very well suited to certain types of home appliances, for example food processors, in which the induction motor drives usually operate at a speed in the range from about 1715 RPM to about 1765 RPM, depending upon the particular design of the induction motor being used. The working tool in a food processor is located in a bowl and is often directly driven from an extension of the motor shaft.
The rotors of well-built single-phase induction motors are relatively heavy, and consequently they have considerable momentum when running at normal operating speed and are capable of coasting for a considerable time after the electrical power to the motor is turned off or interrupted.
Various electromagnetic braking systems have been disclosed for braking the rotation of rotors in food processors when the electrical power is turned off or interrupted, as disclosed in the following U.S. Pat. Nos.: 4,241,302--Benjamin and 4,395,670--Podell. These elctromagnetic braking systems have proved commercially successful.
There is also a friction braking system disclosed for a commutator-type motor, often called a "universal motor" because it will operate on either A.C. (alternating current) or D.C. (direct current) electrical power, in a food processor disclosed in FIGS. 13-18 of U.S. Pat. No. 4,285,473-Williams. Such universal motors usually run at a rate of speed many times faster than an induction motor and they provide considerably less torque output. Therefore, they need a speed-reducing, torque-increasing mechanical transmission when used in a food processor, as will be seen in the above-identified Williams patent. The friction brake system in the Williams patent is located entirely outside of the motor housing and includes a considerable number of mechanical components.